1. Field of the Invention
This invention relates to an improved hermetically sealed container capable of withstanding elevated pressures which is capable of failing in a safe manner when damaged and to a process for making the container. More particularly, it relates to such a container and process which is especially adapted for use as an all plastic seltzer syphon package and for providing such an all plastic seltzer syphon package. Most especially, the invention relates to such a package incorporating an improved hermetic seal which will fail around the hermetic seal without resulting in flying projectiles when the package is damaged and to a process for making the hermetic seal.
2. Description of the Prior Art
The art relating to the ultrasonic bonding of thermoplastic resins is extensive. That art has been summarized in the above related application. In particular, Fesh, U.S. Pat. No. 3,563,822 described a tack welding process in which a tip of an ultrasonic transducer penetrates nearly through a first layer to be bonded to a second layer, through the first layer to an interface between the first layer and the second layer, or beyond the interface into the second layer while producing the tack welds, apparently producing equivalent results under these three conditions.
In the related application, a tack welding process is used to fix a valve insert into a necked opening of a thermoplastic resin container, with a hermetic line seal being formed between the inside surface of the necked opening and the insert as a result of the configuration of the insert and the cross sectional dimension of the necked opening. This tack welding process produces an improved package under high volume manufacturing conditions suitable for a mass produced packaged syphon carbonated beverage.
Such syphon packages must be capable of withstanding pressures of at least about 90 p.s.i., so that all of the carbonated beverage in the package can be dispensed before the pressure in the package drops enough due to increased head space so that it will no longer force remaining liquid through the syphon for dispensing. The provision of a plastic package for such syphon carbonated beverages represents a major safety advance when compared with the traditional thick glass bottles used to package syphon seltzer. This safety advance has the potential to restore the vestigal syphon seltzer industry. Even with the plastic package as described in the above related application, a potential unsafe failure mode remains. If such a package were to be dropped so that the insert in the necked opening strikes a hard surface, failure of the ultrasonic tack welds could allow the insert to be blown out of the necked opening by the high pressure inside the plastic bottle. Such a flying projectile could injure anyone near the container when this type of failure occurs. In fact, some aluminum twist off caps incorporate vent holes to relieve pressure in the event of a failure of the twist off cap which would otherwise send it flying, even at the lower pressures of about 30 p.s.i. employed with conventional packaged soft drinks.